Rocks are all around us - you can see them in the countryside as cliffs or boulders, in the city as parts of buildings and roads and at the seaside as pebbles. Rocks are made up of minerals; some rocks, like limestone, are made up of only a few types of mineral whereas others, like granite, are made up of lots of different minerals which you can sometimes see as crystals.
There are three types of rock; sedimentary, igneous and metamorphic. Sedimentary rocks are made of compacted particles of other rocks or mud which is usually deposited in lakes and rivers as layers. As the layers build up the pressure increases and the sediment is squeezed into rock. Limestone and coal are examples of sedimentary rocks.
Igneous rocks are produced by volcanic processes; lava erupted from volcanoes and magma (molten rock) held in a chamber beneath a volcano cools into igneous rock. Igneous rocks like granite often have big crystals which grow slowly as the magma cools whereas rocks that have formed from erupted lava cool very quickly and have very tiny crystals, giving a glassy appearance.
Metamorphic rocks form when rocks deep in the ground are squeezed during mountain formation, or when they get very hot. The heat, resulting from the squeezing pressure, causes the rock to recrystallise. Marble is a metamorphic rock; it forms when limestone, a sedimentary rock, is squeezed so hard that its crystals reform.

Want to learn more about the formation of rocks? Click here to see a diagram of the rock cycle. If you want to try and identify your own rock specimen using the rock key click here.